Our objective is to understand the mechanisms by which the NHE1 Na/H exchanger and the AE1, AE2, AE3, and PAT1 (Slc4a1-3 and Slc26a6) Cl/HCO3 exchangers affect cardiac Ca handling, contractility, and metabolism. These acid-base transporters comprise a robust system for control of intracellular pH (pHi), but it is unlikely that this is their only function. Na/H exchange coupled with Cl/HCO3 exchange or the bumetanidesensitive NKCC1 Na-K-2Cl cotransporter operating alone mediate pHi-neutral Na-loading, which in turn can affect Ca handling and contractility. Using gene knockout mouse models, we have identified effects on Ca handling, contractility, metabolism, and susceptibility to heart disease for several of these transporters, and have evidence that AE3-mediated Cl/HCO3 exchange and Na-K-2Cl cotransport can partially compensate for loss of the other activity. In Aim 1 we will use cardiac-specific knockouts for AE1 and AE2, both of which are likely to have functional interactions with NHE1, to determine the effects of Cl/HCO3 exchange dysfunction on cardiac performance in vivo. In Aim 2, isolated myocytes and biochemical analyses will be used to determine the mechanisms by which Cl/HCO3 exchangers and NHE1 dysfunction affects Ca handling, contractility, and metabolism in heart. These experiments will test the hypotheses that modulation of sub-sarcolemmal ion concentrations by these transporters regulates both Ca handling and contractility and that NHE1 and PAT1 exert a major regulatory effect on metabolism and insulin sensitivity in heart. In Aim 3 we will test the hypotheses that specific Cl/HCO3 exchange activities contribute to hypertrophy, and that these activities and that of the loop diuretic-sensitive NKCC1 Na-K-2Cl cotransporter also affect cardiac performance and heart failure in genetic cardiomyopathy models. These studies will provide novel information about the physiological functions of this group of poorly understood transporters in heart and yield important insights about the ion transport and signaling mechanisms by which they affect Ca handling, contractility, and metabolism. In addition, these studies will identify transport activities that are cardio-protective, and therefore should not be inhibited, and transport activities that may be inhibited to elicit cardio-protection. Some of this information has important clinical implications, particularly with regard to metabolic abnormalities, including insulin resistance, in heart disease and adverse effects of loop diuretics.